Personal computers come with many standard features, however some features are not shipped with all personal computers. A user can add additional capabilities to a computer by installing additional printed circuit (PC) boards, (sometimes referred to as daughter cards) into the computer. These daughter cards are typically added by installing the daughter cards into edge connectors that are mounted on the main processor board (motherboard) of the personal computer. The daughter cards (102) typically have pads (104) along the edge of the board that make contact with the individual contacts (106) in the edge connector (108). These connections between the pads (104) on the daughter card and the contacts (106) in the edge connectors serve as the electrical connection between the computer motherboard and the daughter cards. The edge connectors make the electrical connection to the plated area, or pad (202), on the daughter card (208) by providing an exerting force (206) (sometimes referred to as the normal force) to the contact (204) to push the contact firmly against the pad (see FIG. 2). Unfortunately edge connectors have a number of problems that affect the reliability of the connections between the pads and the contact points in the edge connector.
One problem is that the pads on the daughter card can get dirty. This can affect the connection in two ways. First, the pads can be covered or splattered with a contaminant that forms a thin film. If the film is not displaced by a wiping action as the daughter card is inserted into the edge connector, the film can prevent the contact from touching the pad and making electrical connection with the pad.
The amount of force or contact pressure between the pad and the contact point is a delicate balance between contact area and the normal force. When the contact area is relatively small with respect to the normal force, the contact pressure is high, and the contact can rip or wear plating from the surface of the pad. When the contact area is relatively large with respect to the normal force, the contact pressure is low, and the contact can fail to displace or wipe off the insulating surface contaminants, resulting in contact failure. Unacceptably high insertion force can result when a connector has a large number of contacts and a high normal force at each contact. Some edge connector applications, for example a test fixture, require a high number of insertion cycles over the life of the connector. In these applications a low normal force is desired to minimize the wear on the contacts and pads to extend the life of the connector. Today, typically a normal force of approximately 10 grams per contact is considered a low normal force and approximately 100 grams per contact is considered a high normal force.
Second, when the dirt on the daughter card is in the form of particles, the particles can wedge between the contact and the pad, lifting the contact away from the pad and preventing electrical connection to the pad. Other problems that can occur with edge connectors include plating defects on the pads, poor alignment of the contacts to the pads, and susceptibility to thermal changes, due to contact movement on the pad surface.
These problems are indicative of a common characteristic of edge card connectors, a single point of contact between the connector and the pad surface on the daughter card. This extremely small single point of contact can be rendered ineffective by plating defects, surface contamination, excessive wear, poor alignment, and motion. The result is that the entire interconnection can fail due to a small problem at a critical point. Making multiple redundant contacts between the connector and the plated surface of the daughter card can reduce these problems. By providing at least two contact points for each connector pin the chance that a random localized particle, film, dust or other contaminant will be able to cause a connector failure has been greatly reduced.
There are a number of ways that multiple redundant contacts can be implemented. One way is to send one signal to two different contacts connected to two different pads. This method can be used without any changes to current connector design. Unfortunately this method reduces the total number of signals that can be sent through the connector. If each signal were sent over two contacts the total number of signals that can be sent through the connector would be cut in half. Sending each signal to two different pads also increases the capacitance for each signal reducing the maximum operational frequency for the connector.
Another method to implement multiple redundant contacts is to cut the end of the contact into two prongs (see FIG. 3). This method creates two contact points (302) on the same pad. By creating two contact points on one pad the number of signals sent through the connector is not reduced. Multiple contacts on one pad also reduce the overall contact resistance. The multiple contacts form a parallel circuit and the resistance of parallel circuits is a function of the resistance per element, divided by the number of elements. Unfortunately when localized surface imperfections are present on the daughter card and one or both of the split contacts snag the imperfection during card insertion, locally high stresses can be inflicted into one or both of the split contacts. This can result in catastrophic contact failure and permanent damage to the connector. Because the connector manufacturer only makes the connector half of the mating pair of connector/daughter card, the connector manufacturer can not prevent this problem by controlling for surface imperfections of the daughter card.
Edge connectors are used in a wide variety of applications in addition to personal computers. The descriptions using personal computers as examples are for clarity of understanding and are not meant to limit the invention to edge connectors in personal computers.
Edge connectors are typically produced with hard tooling. Hard tooling is typically costly to build. An edge connector design with redundant contacts that can be added onto a single point connector would leverage the use of the hard tooling for two connectors. Therefore there is a need for a multiple redundant contact that can withstand surface imperfections during card insertion and can be added onto a single point connector design.